The invention relates generally to methods and apparatus for balancing a rotor and more specifically to methods and apparatus for midlength balancing of a rotor.
A rotating body will not exert any variable disturbing force on its supports when the axis of rotation coincides with one of the principal axes of inertia of the body. This condition is quite difficult to achieve in the normal process of manufacturing since due to errors in geometrical dimensions and non-homogeneity of the material, some irregularities in the mass distribution are always present.
As a result of the above, variable disturbing forces occur which produce vibrations. To remove these vibrations and establish safe and quiet operation, balancing becomes necessary. The importance of balancing becomes especially great in the case of high-speed machines, for example a turbine rotor or electric machinery that operates at speeds typical of turbomachinery. In such cases the slightest unbalance may produce a very large disturbing force.
The proper balancing of certain rotors, particularly for high-speed applications, requires unbalance correction not just in the end regions of the rotor but also in the midlength of the rotor body. Common rotor architectures, especially in high-speed applications, make use of an outer shell of material that is highly stressed. This highly stressed material cannot tolerate the stress concentrations caused by weight-addition or weight-removal features typically used for balancing. Accordingly, the only locations available for balancing rotors of this configuration have traditionally been in the end regions of the rotor where the materials are not as highly stressed. With correction planes available only in the end regions, the speed range over which a rotor can be successfully operated in without rotor dynamic instability is limited.
The typical solution is to design a rotor with high enough stiffness such that the residual unbalance of the unit can be corrected in locations on either end of the rotor body. This effectively restricts the size and speed that a rotor can operate. If a design must operate in a regime where mid-length rotor balancing is required, then sub-component and sub-assembly balancing techniques can be employed to limit the potential unbalance of the full assembly. This approach has limited effectiveness, as it does not address the unbalance caused during the assembly process. The ability to correct the rotor balance at axial locations between the end regions after it is assembled can enable the rotor to operate at higher speeds without experiencing instability issues.
The process by which a typical high-speed rotor is balanced requires access to the balance correction locations. For a traditional balancing scheme, correction masses are added or removed from available end regions. Access to these regions is often restricted by the rotor support structure and the non-rotating machine components. Because of access restrictions, the balancing operations are often conducted before the rotor is assembled to the machine. Future balance correction for machines of this nature require significant disassembly to gain access to the balance correction locations. End-wise access to balance correction locations can enable rotor balancing on a machine that is nearly fully assembled, ideally requiring access to only one end of the rotor. Future balance correction can be enabled in this way without significant machine disassembly.
Accordingly, there is a need for an improved rotor balancing method and apparatus, especially for midlength rotor balancing.